I have a 2006 Hummer H3 Luxury package with about 70,000 miles on it. I am having two problems with it:

1. Whenever I turn off the engine I hear a short popping noise. I'm not sure what it is, but it sounds like it's coming from the engine. It happens right after I turn the ignition off. I installed a K&N intake on it about 7 months ago, not sure if that has anything to do with it. Sometimes I get the check engine light on with the code P0171 for the mass airflow sensor, but that comes on and goes off.

2. Whenever I drive it, it sounds like pennies rattling in a jar. Doesn't matter if I am going straight or turning, I still hear it. When I go faster the sound gets louder too. I have taken it off-roading a couple times, so that is probably what sparked this problem. It's strange because I can hear it perfectly, but when I ask my parents if they hear anything, they don't hear it at all. Could it be failing U-joints or the bearings?

P.S. Is it normal to average 12.2 MPG on mostly highway for around 60 miles total? I'm not lead-footed.

Thanks for your help in advance!